Many species are grown for their ornamental flowers. They are native to the temperate Northern Hemisphere, south into tropical mountains in Ethiopia, Indonesia and New Guinea, and in temperate southern South America.
Some flowering forms of (cultivated) Primula are commonly known as polyanthus (P. elatior hybrids) as opposite to primrose (P. vulgaris hybrids).
The morph phenotype is genetically closely linked to genes responsible for a system of self-incompatibility. That means the pollen from a flower on one morph cannot fertilize another flower of the same morph.
In one morph (termed 'pin') the stamens are short and the pistils are long; in the second morph (termed 'thrum') the stamens are long and the pistils are short. The length of the pistil in one morph equals the length of the stamens in the second morph, and vice versa. The system prevents self-fertilisation.
All sections of the genus Primula have heterostyle species, altogether 354 species out of 419. Since heterostyly is characteristic of nearly all races or species, the system is at least as old as the genus.
- 'closely linked' means close together on the same chromosome.
- Darwin, Charles 1862. On the two forms, or dimorphic condition, in the species of Primula, and on their remarkable sexual relations. Journal of the Proceedings of the Linnaean Society (Botany) 6, 77–96.
- Darwin, Charles 1877. The different forms of flowers on plants of the same species. Murray, London.
- Bruun H.G. 1938. Studies on heterostyle plants 2. Svensk. Bot. Tidskr. 32, 249–260.
- Darlington C. 1958. Evolution of genetic systems, 2nd ed, p120 et seq: The genetic promotion of crossing. Oliver & Boyd, London.